Ophthalmic diseases are most commonly treated by instillation of eye drops with frequencies varying from one or two a day for diseases like glaucoma to as many as ten a day for severe infections. The drug solutions in eye drop bottles can get contaminated during use due to contact of the tip with hands or tears while instilling the drops. In a recent study with 204 glaucoma patients, only 39% were able to instill the eye drops without touching the bottle to the eye surface. There are additional risks of cross-contamination when multiple patients share a bottle, such as in a family or in hospitals. The high potential for the contamination after opening the bottles has led to regulations that require the addition of an antimicrobial agent in multi-dose eye drop formulations. Several preservatives have been researched and used in commercial formulations, including: alcohols, parabens, EDTA, chlorhexidine, and quaternary ammonium compounds. In addition to antimicrobial efficacy, the preservatives require suitable physical properties for incorporation into the formulations, such as chemical and thermal stability, compatibility with the eye drop container and other compounds in the formulation, and, more importantly, negligible toxicity to ocular tissues.
Regulations require that ophthalmic preservatives achieve 1.0 and 3.0 log reduction by days 7 and 14, respectively, along with no increase in survivors from days 14-28 and no increase in survivors for the fungi from day 0 to day 28 after inoculation with 106 colony forming units (cfu)/mL. (Baudouin et al. “Preservatives in Eyedrops: the Good, the Bad and the Ugly”. Progress in Retinal and Eye Research, 2010, 29, 312-34) Due to high efficacy and low corneal toxicity, the quaternary ammonium compounds are preferred preservatives. Benzalkonium chloride:
where a mixture of n being 8, 10, 12, 14, 16, and 18, is the most common choice with n=12 and 14 being the primary homologues. Eye drop formulations require BAK at concentrations ranging from 0.004 to 0.025% (w/w) to achieve the regulatory effectiveness. In spite of the positive safety profile of BAK, achievement of the targeted antimicrobial and antifungal effects is not possible without levels that cause some toxic side effects to the cornea. BAK can cause tear film instability, loss of goblet cells, conjunctival squamous metaplasia and apoptosis, disruption of the corneal epithelium barrier, and damage to deeper ocular tissues.
The potential for ocular damage from the preservatives is particularly high among patients suffering from chronic diseases that require daily eye drop instillations for periods of years to decades, such as glaucoma patients. Several clinical and experimental studies have shown that toxic side effects from preservative free eye drops are significantly lower than from their preserved counterparts. A multicenter cross-sectional epidemiologic study using preservative or preservative-free beta-blocking eye drops showed that patients on preservative free eye drops exhibit significantly fewer ocular symptoms and signs of irritation compared to those using preserved eye drops. (Jaenen et al. “Ocular Symptoms and Signs with Preserved and Preservative-free Glaucoma Medications”, European Journal of Ophthalmology. 2007, 17, 341-9) Preserved glaucoma drug timolol causes significantly higher tear film instability and disruption of corneal barrier function than preservative-free timolol in healthy subjects. (Ishibashi et al., “Comparison of the Short-term Effects on the Human Corneal Surface of Topical Timolol Maleate with and without Benzalkonium Chloride”, Journal of Glaucoma, 2003, 12, 486-90) Similar results were found when comparing preservative-free and BAK-containing carteolol. (Baudouin et al., “Short Term Comparative Study of Topical 2% Carteolol with and without Benzalkonium Chloride in Healthy Volunteers”, British Journal of Ophthalmology. 1998, 82, 39-42) Goblet cell loss and increased cytoplasmic/nucleus ratio, two characteristics of dry eye disease, have been shown to occur when using BAK containing tear substitutes. (Rolando et al., “The Effect of Different Benzalkonium Chloride Concentrations on Human Normal Ocular Surface”. The Lacrimal System, Kugler and Ghedini, New York 1991, 87-91) A significant reduction in Schirmer test values was observed for subjects receiving BAK eye drops compared with subjects not receiving therapy. (Nuzzi et al., “Conjunctiva and Subconjunctival Tissue in Primary Open-angle Glaucoma after Long-term Topical Treatment: an Immunohistochemical and Ultrastructural Study”, Graefe's Archive for Clinical and Experimental Ophthalmology, 1995, 233, 154-62) Patients using preserved eye drops and experiencing toxicity symptoms, such as allergy, blepharitis or dry eye, experienced rapid improvements upon switching to preservative-free formulations. Such studies suggest a role of preservatives in the preponderance of dry eye symptoms in glaucoma patients, who typically use multiple drugs with multiple instillations each day.
BAK is considered a ‘necessary evil’ for prevention of microorganism growth in the bottles while displaying toxic effects on the ocular tissue. The industry has taken a few approaches to solve this problem. One approach is to develop more efficacious glaucoma therapies, such as: use of prostaglandins that require instillation of only one eye drop each day; and combinations that contain multiple drugs in the same formulation to eliminate instillation of multiple eye drops. Nevertheless, both of these approaches still permit a cumulative effect to preservatives over long periods of time. Furthermore, only a few combination products are available, generally combinations from a single manufacturer.
A second approach is to provide single dose packages, and several glaucoma formulations are now available as preservative free single doses. While this approach can eliminate exposure to preservatives, in addition to increasing manufacturing costs and the environmental impact of packaging, single dose formulations contain about 0.3 to 0.4 mL of formula, which is significantly more than the typical eye drop volume of 30 Lμ, leading to wastage or possibly misuse of the same package for multiple days. This approach can suffer if bacterial contamination occurs prior to packaging.
Another approach is to replace BAK with a less toxic preservative, such as: Purite®, a stabilized oxychloro complex; and Sofzia®, which is composed of boric acid, propylene glycol, sorbitol, zinc chloride and polyquaternium compounds, some of which are used in contact lens care solutions. While these alternatives may be promising, no data on long term impact from use of these preservatives is available, and consistent use of these preservatives over extended periods of years may well prove them toxic.
The solution in a bottle is typically contaminated during the instillation of the eye drops due to the contact of the bottle tip with the eye surface, contact of the tip with hands, or both. As the eye drop detaches from the bottle, a small volume of liquid remaining at the tip is sucked back, which can take the bacteria into the bottle, leading to the contamination. An ABAK® (Laboratoires Théa, France) design introduces a 0.2 μm filter at the top of the bottle to filter out bacteria from the re-entering solution, thereby preventing contamination. Though effective, this approach does not protect against contamination prior to packaging. Also the 0.2 μm filter could require additional pressure to push the drops, making drop instillation difficult, particularly for the elderly. Additionally, any leak in the filter or bacteria transport through the pores could allow the formulation in the bottle to get contaminated. It is also not clear whether this design can protect against growth of bacteria trapped in the filter. The COMOD® (Ursapharm, Germany) system combines an air free pump and an inner lining that retracts as the liquid is pushed out to avoid contamination of the contents of the bottle. While this design is innovative and useful, its complexity and increased cost are major concerns. As with ABAK®, COMOD® cannot protect against any microorganisms introduced due to errors in the manufacturing processes causing loss of sterility. This makes the filling of these devices complicated because sterility is essential at each step.
U.S. Pat. No. 5,080,800 teaches a process for removing components from solutions, including preservatives from eye-drops. The process involves the use of ion exchange resins to selectively remove ocular preservatives. Ion exchange resins have not been tested extensively for biocompatibility and cytotoxicity and inherently are non-selective, adsorb ionic drugs as readily as any ionic preservative such as BAK. The hydraulic permeability of these resins is not addressed although this characteristic is critical for devices that allow formation of drops without excessive pressure. U.S. Pat. No. 5,080,800 also does not teach on the importance of ensuring that the filters are designed to resist growth of microorganisms that may remain trapped. U.S. Pat. No. 5,080,800 does not teach on the possibility of dilution of the BAK concentration in the formulation because of draining of the BAK free formulation from the filter into bottle after each eye drop instillation. Hence a practical way of retaining the beneficial behavior of preservatives while avoiding their toxic effects in the eye remains a need.